jou rn al h om ep age: www.elsev ier .com/ locate /apsusc
urface modification to produce hydrophobic nano-silica particlessing sodium dodecyl sulfate as a modifier
ing Qiao, Yong Liang, Ting-Jie Wang ∗, Yanping Jiangepartment of Chemical Engineering, Tsinghua University, 100084, Beijing, China
r t i c l e i n f o
rticle history:eceived 10 September 2015eceived in revised form 7 December 2015ccepted 15 December 2015vailable online 17 December 2015
eywords:ano-silica particleurface modification
a b s t r a c t
Hydrophobic silica particles were prepared using the surfactant sodium dodecyl sulfate (SDS) as a mod-ifier by a new route comprising three processes, namely, aqueous mixing, spray drying and thermaltreatment. Since SDS dissolves in water, this route is free of an organic solvent and gave a perfect dis-persion of SDS, that is, there was excellent contact between SDS and silica particles in the modificationreaction. The hydrophobicity of the modified surface was verified by the contact angle of the nano-sizedsilica particles, which was 107◦. The SDS grafting density reached 1.82 nm−2, which is near the highestvalue in the literature. The optimal parameters of the SDS/SiO2 ratio in the aqueous phase, process tem-perature and time of thermal treatment were determined to be 20%, 200 ◦C and 30 min, respectively. The
odium dodecyl sulfateydrophobicrafting
grafting mechanism was studied by comparing the modification with that on same sized TiO2 particles,which indicated that the protons of the Brønsted acid sites on the surface of SiO2 reacted with SDS togive a carbocation which then formed a Si–O–C structure. This work showed that the hydrophilic surfaceof silica can be modified to be a hydrophobic surface by using a water soluble modifier SDS in a newmodification route.
Nano-silica particles are widely used in many products such asubbers, resins, pigments and paints [1–4]. However, due to theigh surface energy and abundant hydroxyl groups on the silicaurface, these nano-particles tend to agglomerate, which result in
weak affinity with the polymer matrix and poor performancef the composite material in applications. Therefore, a modifica-ion to produce a hydrophobic surface is necessary to improve theispersion and compatibility of nano-silica particles in an organicatrix.The most commonly used organic modifier is the silane cou-
ling agent [5–8]. Silane reacts with OH groups on the silica surfacey a condensation reaction, sometimes with the help of a catalyst9,10]. The modification with silane is often done in a non-aqueousolvent such as toluene or hexane [11–13] to avoid the hydrox-lation and self-condensation of silane in aqueous solution. Otherrganic modifiers such as alkanoic acids [14–16] and long chain
lcohols like octanol and dodecanol [17,18] also have to be usedn an organic solvent because they are water insoluble. The solventas to be removed and recycled, and this can easily cause pollution.
Also, hydrophilic particles agglomerate in a hydrophobic solvent,resulting in the diffusion limitation of the modification process. Gasphase modification [19,20] avoids the use of an organic solvent butit also suffers from diffusion limitation in the modification processas the hydrophilic nano-particles exist as agglomerates in the gasphase.
Sodium dodecyl sulfate is a common surfactant. It has beenused to prepare surfactant-coated adsorbents for solid phaseextraction because it can adsorb on an oppositely charged metaloxide surface such alumina [21,22], Fe3O4 [23,24] and ferri-hydrite [25] to form hemimicelles and admicelles. This causeshydrophobic organic compounds to concentrate in the hydropho-bic phase to effect the extraction. However, it has not yet beenused as a chemical modifier to produce a hydrophobic surface forparticles.
In this paper, a new route for preparing hydrophobic silicawithout any organic solvent was developed using sodium dodecylsulfate (SDS) as the modifier. Since SDS dissolves in water, it canbe well mixed with the SiO2 particles in an aqueous solution. Thehydrophobic modification of the silica particles was completedby a subsequent spray drying and thermal treatment. The graft-
ing density and contact angle were determined, and the optimalparameters for the SDS/SiO2 ratio, reaction temperature and timewere determined. The mechanism of the surface modification isdiscussed.
Commercial nano-silica particles (Luodiya Silica White Co. Ltd.,ingdao, China) from an aqueous precipitation process were used.he BET surface area of the nano-silica particles was 163 m2 g−1,hich was measured by a surface area analyzer (Autosorb-iQ,uantachrome Instruments USA). The average diameter of therimary particles was 20 nm. The TiO2 particles (AEROXIDE P25,egussa, Germany) have 80% anatase and 20% rutile crystal struc-
ure with an average primary diameter of 21 nm. The TEM imagesf the SiO2 and TiO2 nanoparticles used are shown in Fig. 1. Allhe chemicals, namely, sodium dodecyl sulfate (SDS) (Sinopharmhemical Reagent Co. Ltd., China), anhydrous ethanol (Modern Ori-ntal Technology Development Co. Ltd., Beijing, China), methylrange (Modern Oriental Technology Development Co. Ltd., Beijing,hina), Na2SiO3 (Beijing Chemical Works, China), H2SO4 (Bei-
ing Chemical Works, China), and NaOH (Beijing Chemical Works,hina), were analytical reagent (AR) grade.
.2. Modification process
The modification process was adopted from our previous work26]. It comprised three processes, namely, aqueous mixing, sprayrying and thermal treatment. In the aqueous mixing process, an
mount of SDS was added to 150 mL deionized water in a conicalask under magnetic stirring until the SDS was dissolved com-letely. Then 4 g SiO2 particles was added to the solution. The massatio of SDS/SiO2 was chosen to be from 0 to 75 wt%. The slurry
ig. 1. Morphology of SiO2 and TiO2 particles (a) TEM image of SiO2; (b) TEM imagef TiO2.
ience 364 (2016) 103–109
was kept stirred for 1 h at room temperature to give a good disper-sion. Then the slurry was dried in a spray dryer. The agglomeratedparticles formed were collected and given a thermal treatment inwhich the agglomerated particles were put in a sealed steel pipeand heated in a muffle furnace. The process temperature used wasin the range of 150–250 ◦C, and the processing time used was in therange of 0–2 h. After the thermal treatment, the product (marked assample SiO2-T) was thoroughly washed with anhydrous ethanol toremove unreacted SDS. The ethanol washing process was adoptedfrom our previous work [26]. Since SDS is slight soluble in anhy-drous ethanol, the TG curves of the spray dried sample after ethanolwashing and water washing were compared to confirm that theunreacted SDS was completely removed by the rigorous ethanolwashing. The sample after ethanol washing was dried at 80 ◦C for12 h, and marked as SiO2-T-EW. For indicating the samples clearly,the key parameters were used to name the samples, e.g., the sam-ple after thermal treatment at 200 ◦C for 30 min and with ethanolwashing was marked as SiO2-T-200-30-EW.
2.3. Preparation of silica coated TiO2 particles
Modification of TiO2 and silica coated TiO2 particles were carriedout for comparing how the nature of the particle surface affectedthe modification. The TiO2 particles were about the same averagesize as the SiO2 particles. The surface coating of the TiO2 parti-cles was conducted as described in our previous works [27,28]. Itwas conducted in a flask with the temperature and pH monitoredonline with a thermometer and pH meter. The TiO2 particles weredispersed in deionized water at a concentration of 50 g/L (10 g TiO2particles + 200 mL water). The temperature was controlled at 60 ◦Cby a constant temperature bath. Then 1 mol/L Na2SiO3 solutionand H2SO4 solution were simultaneously titrated into the suspen-sion of TiO2 particles. The suspension was stirred vigorously andcontrolled at pH 9 by adjusting the titration rate of the H2SO4solution using a peristaltic pump while the titration rate of theNa2SiO3 solution was kept constant. The thickness of the coatedfilm was designed to be 2 nm, which corresponded to the silicacoating amount of 30 wt% (SiO2/TiO2). After titration, the suspen-sion was aged for 2 h while stirred. Then, the coated TiO2 particleswere filtrated and washed repeatedly, and dried at 105 ◦C for 24 h.The product obtained was marked as SiO2@TiO2.
2.4. Characterization
The morphologies of the TiO2 and SiO2@TiO2 particles werecharacterized by high resolution transmission electron microscope(HRTEM, JEM-2011, JEOL Co., Tokyo, Japan). The amount of sili-con in sample SiO2@TiO2 was determined by energy dispersiveX-ray spectroscopy (EDS). The grafted amount on the samples afterethanol washing was determined by a thermogravimetric analyzer(TGA/DSC 1, Mettler toledo, Swiss). In the TG analysis, the heatingrate was 20 K/min from 30 ◦C to 1000 ◦C under a flow of nitro-gen. The contact angle of the samples was measured by a contactangle analyzer (HARKE-SPCA, Beijing Harke, China). The character-istic groups on the sample were determined by a FTIR spectrometer(Nexus 670, Nicolet, USA) using KBr as the matrix. The sulfur andcarbon contents in the modified silica samples were determined byan elemental analyzer (EA, Vario EL III, Elementar, Germany) with aprecision of 0.01%. The crystal structure of the samples was deter-mined by a X-ray diffraction analyzer (XRD, D8-Advance, Brucker,Germany) over the range of 10–90◦ (2� angle) at a scan speed of
5◦/min. The chemical state of the sample surface was analyzed by X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM, Ulvacphi,Japan). All binding energies were calibrated by reference to the C1s peak, which was fixed at 284.8 eV.
B. Qiao et al. / Applied Surface Science 364 (2016) 103–109 105
.1. Hydrophobicity of the SDS modified silica particles
The water contact angles of the pure silica particles, spray driedample and thermal treated samples before and after ethanol wash-ng (SiO2-T-200-30 and SiO2-T-200-30-EW) are shown in Fig. 2. Theure silica particles were hydrophilic with a water contact angle of4◦. The spray dried sample with SDS presence had a water con-act angle of 17◦, which was still hydrophilic. However, the waterontact angle of sample SiO2-T-200-30 and sample SiO2-T-200-30-W were 104◦ and 107◦, respectively. This showed that the silicaurface was successfully modified from hydrophilic to hydropho-ic. This also showed that a reaction occurred between SDS and theilica particles in the thermal treatment process.
Fig. 3 shows the FTIR spectra of SDS, pure silica particles andiO2-T-200-30-EW. In the spectrum of SiO2-T-200-30-EW, thebsorption peaks at 3400 and 1630 cm−1 were assigned to thetretching and bending vibration modes of the hydroxyl groupsf physical adsorbed water molecules and silanol groups. Thebsorbance peak at 1106 cm−1 was assigned to Si–O–Si groups. The923 and 2852 cm−1 peaks were assigned to the stretching vibra-ion of CH2, and the 1463 and 1387 cm−1 peaks to the deformingibration of C–H, which were not found in the spectrum of the pureilica particles but were in the spectrum of SDS, indicating that SDSas successfully grafted onto the silica surface.
The TG and dTG curves of the pure silica particles, and SiO2--200-30 and SiO2-T-200-30-EW samples are shown in Fig. 4. For
he pure silica particles, the weight loss in the temperature rangef 30–190 ◦C was from the removal of molecular water adsorbedn the silica surface [29]. When the temperature was higher than90 ◦C, the weight loss was from dehydroxylation, which was not
400 0 350 0 3000 2500 2000 1500 1000 5000
20
40
60
80
100
Wavenumber (cm-1)
SDS Si O2-T-200-30-E W SiO2
2923 c m
2852 cm
1463 cm3400 cm1630 cm
1106 cm
1387 cm
(a)
(b)
(c)
ig. 3. FTIR spectra of (a) SDS, (b) SiO2-T-200-30-EW and (c) pure SiO2 particles.
T ( C)
Fig. 4. TG curves of SiO2, SiO2-T-200-30 and SiO2-T-200-30-EW.
significant. For the SiO2-T-200-30 sample, two obvious weight losspeaks at 190–360 ◦C and 360–600 ◦C were observed. For SiO2-T-200-30-EW, the weight loss at 190–360 ◦C was small while theweight loss at 360–600 ◦C was quite obvious. This indicated thatthe weight loss at 190–360 ◦C for SiO2-T-200-30 was mainly fromphysical adsorbed SDS, which was removed by ethanol washing.The weight loss at 360–600 ◦C was from the decomposition of thegrafted SDS on the silica surface [26,30], indicating that SDS waschemically bonded on the silica surface. The high decompositiontemperature of 360–600 ◦C was much higher than the decompo-sition temperature of SDS (220 ◦C), indicating that the SDS wasgrafted onto the silica surface. The weak peak at around 300 ◦Cof sample SiO2-T-200-30-EW was inferred to be from the strongadsorption of SDS on the silica surface.
The grafting density of SDS on sample SiO2-T-200-30-EW wascalculated from the weight loss at 360–600 ◦C. It was defined asthe number of SDS molecules grafted onto the silica surface persquare nm. The calculation method was illustrated in our previouswork [26]. This gave 1.82 nm−2 from the TG result. Earlier reports[19,31] suggested that a grafting density of 1.6–2 nm−2 is the maxi-mum surface coverage for a monolayer of the silane coupling agentgrafted onto porous silica particles. Our measured grafting densityof 1.82 nm−2 for SDS was consistent with the value for the silanecoupling agent, indicating that an effective chemical modificationwas achieved in this process.
3.2. Effects of the SDS/SiO2 ratio on the modification
The effects of the SDS/SiO2 ratio from 0 to 75 wt% on the modifi-cation were investigated. The process temperature and time were
106 B. Qiao et al. / Applied Surface Science 364 (2016) 103–109
Fig. 5. TG curves of SDS-modified samples SiO2-T-200-30 (a) and SiO2-T-200-30-EW (b) at different SDS/SiO2 ratios.
Table 1Weight loss of SDS-modified sample SiO2-T-200-30-EW with different SDS/SiO2
ratios.
SDS/SiO2 ratio, wt% 0 7.5 15 20 30 75
smestittwttoSitswkaSSTS
Table 2Weight loss of SDS-modified sample SiO2-T-EWs using different process tempera-tures for 30 min with the SDS/SiO2 ratio at 20 wt%.
Processingtemperature, ◦C
Room tem-perature
150 170 200 220 250
Weight loss at360–600 ◦C, %
1.96 1.96 2.02 4.54 4.87 4.48
Contact angle,◦ 17 15 67 107 107 *
* Cannot be tableted.
Table 3Weight loss of SDS-modified sample SiO2-T-EW at 200 ◦C for different process timeswith the SDS/SiO2 ratio at 20 wt%.
Weight loss at 360–600 ◦C, % 1.45 2.50 5.63 7.45 6.42 7.10Weight loss at 30–190 ◦C, % 4.79 3.75 3.25 3.04 2.83 2.37
et at 200 ◦C and 30 min, respectively. The weight losses of the ther-ally treated samples at different SDS/SiO2 ratios before and after
thanol washing are shown in Fig. 5. Before ethanol washing, ashown in Fig. 5(a), as the SDS/SiO2 ratio increased from 0 to 75 wt%,he weight loss at 190–600 ◦C increased from 2.29% to 30.5%, whichndicated that a high SDS/SiO2 ratio gave more SDS absorbed onhe silica surface after the spray drying process, and resulting inhe increased weight loss. However, most of the unreacted SDSere removed by ethanol washing as shown in Fig. 5(b), where
he weight loss peaks at 190–360 ◦C had disappeared. Table 1 giveshe weight loss of SDS modified SiO2-T-EWs sample in the rangesf 360–600 ◦C and 30–190 ◦C at different SDS/SiO2 ratios. As theDS/SiO2 ratio changed from 0 to 20 wt%, there was an obviousncrease of weight loss at 360–600 ◦C from 1.45% to 7.45%, but ashe ratio was further increased to 30 wt% and 75 wt%, there was alight decrease of the weight loss. The maximum grafted amountas obtained when the SDS/SiO2 ratio was 20 wt%. This was then
ept constant in the following optimization. It was probable thatt too high SDS/SiO2 ratio, competition between the formation of
DS micelles in the solution and SDS adsorption on the surface ofiO2 particles occurred, causing the decrease in the grafted amount.he weight loss at 30–190 ◦C showed an obvious decrease as theDS/SiO2 ratio increased. This was because adsorbed SDS occupied
Processing time, min 10 30 60 120
Wight loss at 360–600 ◦C, % 2.61 4.54 4.21 4.14
the hydroxyl sites and decreased the water molecules adsorbed onthe SiO2 particles.
3.3. Effects of process temperature and time
The effects of process temperature and time during the thermaltreatment on the weight loss and contact angle of the modifiedparticles were investigated. Table 2 shows the weight loss andcontact angle of SDS-modified sample SiO2-T-EWs at different pro-cess temperature for 30 min with the SDS/SiO2 ratio at 20 wt%. Theweight losses at 360–600 ◦C for the sample processed at 150 ◦Cand 170 ◦C were nearly the same as the sample without a ther-mal treatment. Their contact angles were 15◦ and 67◦, respectively,indicating the hydrophilicity of the surface. As the process temper-ature was increased to 200 ◦C and 220 ◦C, the weight loss of thesamples increased to 4.54% and 4.87%, and their contact anglesincreased to 107◦, indicating the hydrophobicity of the surface.After the processing at 220 ◦C, the sample color turned slightlyyellow, which was assigned to SDS decomposition by referring tothe TG curve. When the process temperature was further increasedto 250 ◦C, the SDS decomposition became obvious and the weightloss of the sample decreased. Table 3 shows the weight loss ofSDS-modified sample SiO2-T-EWs at 200 ◦C for different processingtime. The maximum weight loss was obtained at the processingtime of 30 min, which gave the highest quantity of SDS that modi-fied the silica surface. Therefore, the optimal process temperatureand time in the thermal treatment were determined to be 200 ◦Cand 30 min, respectively.
3.4. Effects of surface hydroxyl on the modification
As reported in the literature [32–34], the chemical modifica-tion of inorganic oxide particles occurs by the reaction between thehydroxyl group on the particle surface and the organic modifier. Inorder to compare the effect of different surfaces on the modifica-tion, TiO2 nano-particles with an average diameter of 21 nm (nearlythe same size as that of the silica particles) were modified with thesame process. Silica coated TiO2 particles, namely, SiO2@TiO2, werealso modified for comparison. The surface morphology of the TiO2and SiO2@TiO2 particles, and the elemental composition on the sur-face of the SiO2@TiO2 particles are shown in Fig. 6. A continuous anddense silica film with a thickness of 2 nm was coated on the surfaceof the TiO2 particles, indicating that the SiO2@TiO2 particles havethe properties of a silica surface. The semi-quantitative EDS analy-sis indicated that the SiO2@TiO2 particles contained a Si component
of 11 wt%. Fig. 7 shows the TG curves of the TiO2, SiO2@TiO2 andSiO2 particles. Their weight losses at 190–800 ◦C gave the hydroxylgroup quantity on the surface as 0.77, 0.97, and 2.89%, respectively.After the coating of the silica, the hydroxyl amount on the surface
B. Qiao et al. / Applied Surface Science 364 (2016) 103–109 107
Fig. 6. Morphology and composition of TiO2 and SiO2@TiO2 particles.(a) TEM image of SiO2@TiO2; (b) composition of SiO2@TiO2 from EDS
0 200 40 0 60 0 800 100 090
92
94
96
98
100
Wei
ght l
oss (
%)
o
Ti O2
SiO2@TiO2
SiO2
0.0000
-0.000 2
-0.0004
-0.000 6
-0.0008
-0.001 0
dTG
ot
TwFitp3
0 200 400 600 800 10 0080
85
90
95
100
Wei
ght l
oss (
%)
T (oC)
TiO2-T-200-30-E W SiO2@TiO2-T-200-30-EW SiO2-T-200-30-EW
0.000 0
-0.0005
-0.0010
-0.0015
d TG
Fig. 8. TG curves of SDS-modified TiO2, SiO2@TiO2 and TiO2 particles after ethanolwashing.
Table 4Weight loss and contact angle of SDS-modified SiO2, SiO2@TiO2 and TiO2 particles atthe process temperature of 200 ◦C for 30 min with the SDS/particle ratio at 20 wt%.
SiO2-T-200-30-EW
SiO2@TiO2-T-200-30-EW
TiO2-T-200-30-EW
Weight loss at 7.45 2.78 0.71
T ( C)
Fig. 7. TG curves of TiO2, SiO2@TiO2 and SiO2 particles.
f the SiO2@TiO2 particles was slightly increased to be a little morehan that on the TiO2 particles.
Using the same modification procedure as the SiO2 particles, theiO2 and SiO2@TiO2 particles were modified at 200 ◦C for 30 minith a SDS/particle ratio at 20 wt%. The TG curves are shown in
ig. 8. The weight loss at 360–600 ◦C and contact angle are listed
n Table 4. The data showed that the SDS modified SiO2@TiO2 par-icles had obvious hydrophobicity, while the SDS modified TiO2articles was still hydrophilic. The weight loss of SiO2-T-200-0-EW and SiO2@TiO2-T-200-30-EW were 7.45% and 2.78%,
360–600 ◦C, %Contact angle,◦ 107 96 16
respectively. Their contact angles were 107◦ and 96◦, respectively.However, the weight loss of TiO2-T-200-30-EW was only 0.71%and the contact angle was only 16◦, indicating that there was lessreaction between SDS and the hydroxyl on the surface of the TiO2particles.
3.5. Reaction mechanism of SDS on the silica surface
The hydroxyl quantities on the surface of the TiO2 andSiO2@TiO2 particles, which were 0.77% and 0.97%, were notobviously different, but their SDS modified samples were quite dif-ferent, being hydrophilic and hydrophobic, respectively, as shownin Table 4. This observation was used to infer that the difference wasdue to the difference in the hydroxyl acidity on the SiO2 and TiO2particle surfaces, which has a key role in the modification. Earlierresearchers [33–35] have reported that the SiO2 particle surface hasboth Brønsted and Lewis acid sites, while the TiO2 particle surfaceonly has Lewis acid sites. Only the Brønsted acid sites can provideprotons for the reaction between the surface silanol groups andmodifier.
As reported in the literature [36–38], SDS undergoes hydroly-sis to yield dodecanol and sodium hydrogen sulfate when heated.It was inferred that in this reaction, the dodecanol produced inthe thermal treatment process is attacked by the protons of theBrønsted acid sites on the surface of the SiO2 particles to gener-ate a carbocation. The carbocation then reacts with the negativelycharged Si–O moiety to form a Si–O–C structure and give thehydrophobic modification. The formation of the Si–O–C structurewas characterized by the Si 2p spectra in the XPS analysis as shownin Fig. 9. The binding energies of the Si 2p in SiO2 particles was103.8 eV, while for the sample SiO2-T-200-30-EW, it was shiftedto 103.5 eV, which was assigned to the formation of the Si–O–Cstructure [39–41]. The sodium hydrogen sulfate was further con-
verted to Na2SO4 and H2SO4 during the ethanol washing. H2SO4dissolves in ethanol while Na2SO4 is insoluble in ethanol and cannotbe removed by ethanol washing. Fig. 10 shows the XRD spec-tra of the SiO2-T-200-30-EW and SiO2 particles. The spectrum of
108 B. Qiao et al. / Applied Surface Sc
100 101 102 103 104 105 106 1070
10000
20000
30000
40000
50000
60000In
tens
ity
B.E.(eV)
Si O2
Bac kground Si O2-T-2 00-30- EW Bac kground
Fig. 9. Binding energies of Si 2p of the samples SiO2 and SiO2-T-200-30-EW.
10 20 30 40 50 60 70 80 90
Inte
nsity
2θ
Si O2
Si O2-T-200-30-EW
Na2SO4 (JCPDS 79-1553)
Fig. 10. XRD spectra of modified SiO -T-200-EW, SiO particles and Na SO .
SNttmottme
2eSTcwcwisoipa
[
[
[
[
[
[15] Z. Hu, Y. Deng, Superhydrophobic surface fabricated from fatty acid-modifiedprecipitated calcium carbonate, Ind. Eng. Chem. Res. 49 (2010) 5625–5630.
[16] X. Wu, L. Zheng, D. Wu, Fabrication of superhydrophobic surfaces frommicrostructured ZnO-based surfaces via a wet-chemical route, Langmuir 21
2 2 2 4
iO2-T-200-30-EW contains the XRD characteristic peaks ofa2SO4 that corresponded well with those of the standard spec-
rum. The weight loss at 900 ◦C for sample SiO2-T-200-30-EW inhe TG curves in Fig. 8 also indicated the existence of Na2SO4 as its
elting point is 884 ◦C. The presence of H2SO4 in the supernatantf the washing ethanol was detected by a 0.1% methyl orange solu-ion. The color of the methyl orange solution changed from orangeo red after the addition of the supernatant, while the color of the
ethyl orange solution with the addition of same volume of purethanol was still orange.
For further confirmation, the SDS-modified sample SiO2-T-00-30-EW was washed by a mixed solution of water andthanol (V/V = 3/2) to remove Na2SO4. This sample was marked asiO2-T-200-30-EW-W. After the thorough washing, sample SiO2--200-30-EW-W was still hydrophobic. The carbon and sulfurontent of SiO2-T-200-30-EW and SiO2-T-200-30-EW-W samplesere determined by an elemental analyzer. The carbon content had
hanged from 6.06% to 5.54% after the washing. The slight decreaseas caused by the hydrolysis of the grafted Si–O–C bond as this
s easy to break [31,33]. As for the sulfur content, since Na2SO4 isoluble in water, a trace of sulfur with a content of 0.765% existingn sample SiO2-T-200-30-EW was totally removed by the wash-ng with a mixed solution of water and ethanol since the sulfureak of SiO2-T-200-EW-W was below the detection limit of the
nalyzer.
ience 364 (2016) 103–109
4. Conclusions
Hydrophobic modification of nano-silica particles was achievedusing the common surfactant sodium dodecyl sulfate (SDS) as amodifier by a new route comprising three processes, namely, aque-ous mixing, spray drying and thermal treatment. This route doesnot use an organic solvent and gives a perfect dispersion, whichindicated an excellent contact between SDS and the SiO2 nano par-ticles for the modification reaction. The optimal SDS/SiO2 ratio, andprocess temperature and time in the thermal treatment to givea high density of grafted SDS on the silica particle surface weredetermined to be 20 wt%, 200 ◦C and 30 min, respectively. Underthis condition, the hydrophobic nano-silica particles had a con-tact angle of 107◦. The grafted density reached 1.82 nm−2, which isnear the highest value in the literature [19,31]. Brønsted acid siteson the surface of the SiO2 particles supplied protons to react withSDS, which generated a carbocation that formed a Si–O–C struc-ture. Thus the hydrophilic surface of silica can be modified into ahydrophobic surface by using the water soluble modifier SDS in anew modification route.
Acknowledgments
The authors wish to express their appreciation for the financialsupport of this study by the National High Technology Research andDevelopment Program (863 Program, No. 2012AA062605) and theNational Natural Science Foundation of China (NSFC No. 21176134).
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